1. Field of the invention
The present invention relates to an optical recording medium that switches either the irradiation time or the irradiation power of a laser beam in many levels in correspondence to data used in the recording, irradiates the recording layer with the laser beam and then records the data in multiple levels.
2. Related Art
A great deal of research has been carried out related to methods to record multiple pieces of data in signals with identical length by switching the depth of a regeneration signal (modulation factor of reflection signal) in many levels in contrast to methods to record data by changing the length of a regeneration signal (length of the modulated part of the reflection signal) in many levels with conventional optical recording mediums such as CD-R or DVD-R which are provided with recording layers and reflecting films, in this order, on an optically transparent substrate.
According to this optical recording method, because it is possible to record multiple pieces of data in the direction of depth compared to when binary data is recorded depending on the presence or absence of only a pit, the amount of signals assigned to fixed lengths can be increased. Consequently, optical recording methods which use holographs or optical recording mediums with multiple recording layers have been proposed in order to improve the linear recording density.
Hereupon, a case wherein data is recorded in many levels using a depth variation of the reflectance is called multilevel recording.
In this type of multilevel recording the recording marks must be shortened in order to improve the recording density.
Multilevel recording is, however, difficult when attempting to reduce the recording marks smaller than the beam diameter of a converged laser used for recording and reading.
For example, Japanese Patent Laid-Open Publication No. Hei. 10-134353 describes a method in which the quantity of laser light is adjusted in order to record multiple levels. In this method a regeneration signal is formed by differences in the reflection of the recording part and the non-recording part when the recording medium is a dye film or a phase-changing film. Consequently, in the method in Japanese Patent Laid-Open Publication No. Hei. 10-134353, the non-recording level and the recording level depend on a relationship of whether or not a recording exists and are not suitable for recording in many levels. Stated more clearly, nothing exists in the intermediate state between recording and non-recording for a phase-changing film or a dye film.
Up to the present the reason why multilevel recording in many levels was possible by means of modulating the quantity of laser light using a dye film or a phase-changing film as the recording medium was mainly due to the fact that the widths of the recording marks were changed by changing the power of the laser.
A converging beam normally forms a Gaussian distribution although when the recording film is a dye film or a phase-changing film, the recording is performed on the portion that exceeds a certain threshold value. Changing the power of the laser changed the spot size of the converging beam that can record which in turn changed the width of the recording marks.
If, however, the length of the recording marks are shortened to increase the recording density, it becomes difficult to perform multilevel recording in many levels, in particular in five levels or more, using a method that modulates the power of the laser to change the recording mark width. In other words, changing the recording power makes it difficult to change the reflection level during a regeneration in five levels or more,
Normally, the diameter of the converging beam is expressed by Kxcex/NA (K; constant, xcex: laser wavelength, NA: numerical aperture). Normal values for a pickup used in a CD are xcex=780 nm, NA=0.45 with a diameter of approximately 1.6 xcexcm. For this case, if the recording mark length was 1.6 xcexcm or less, it is difficult to perform multilevel recording in five levels or more using a conventional method that changes the laser power.
Further, there is an example of an optical recording medium as disclosed in Japanese Patent Laid-Open Publication No. Hei. 1-182846 wherein the absorbance of the reactive material in the recording layer changes as a digital value when a quantity of incident light is supplied as a digital value.
The absolute value of the absorbance change is presumed to be very small for this optical recording medium however and has not yet attained practical use.
Furthermore, an optical recording method is disclosed in Japanese Patent Laid-open Publication No. Sho. 61-211835 in which the intensity or irradiation frequency of the irradiation light irradiating a photochromic material is changed in order to record at different arbitrary coloring density states.
In this optical recording method there is a problem in which the coloring density state cannot be read in live levels of more when irradiating and scanning laser light.
The rapid occurrence of thermal decomposition of dyes in optical recording mediums used as a recording materials which use dye as the principal material was considered to be favorable in conventional recording methods. This is because the signal quality is better due to the clear boundary between the recording part and the non-recording part.
If, however, there is rapid decomposition of the dye material during multilevel recording, decomposition of the dye will suddenly begin when a specified laser irradiation time or irradiation power is exceeded making it difficult to control the recording in many levels required for multilevel recording.
The inventor has discovered that it is possible to perform multilevel recording in five levels or more by means of changing 7:4 the irradiation time or the irradiation power of the laser when the recording mark length is shorter than the diameter of the converging beam. The inventor has also discovered that a dye material that changes gradually is more suitable as a material for a recording film than a phase-changing material that changes quickly from non-recording to recording following temperature increases in the laser irradiation.
Furthermore, the inventor has found out it is possible to favorably perform multilevel recording by stipulating the thermal decomposition characteristics of the dye material.
At this point the thermal energy absorbed by the recording film will grow larger in line with lengthening of the laser irradiation time and/or increases in the laser irradiation power. If the thermal energy exceeds a certain threshold value, the dye will decompose and deteriorate and recording will be performed on the recording film. Excessive thermal energy that exceeds the threshold value passes through the reflecting film and diffuses around the edge. For example, if the diffusion of the thermal energy is insufficient for an optical recording medium such as a CD-R, unfavorable effects will occur such as deformation of the substrate or the guide tracks cut into the substrate.
Taking the above-mentioned issues into consideration, an object of the present invention is to provide an optical recording medium that utilizes a widely used optical recording medium, such as CD-R, to perform multilevel recording in many levels and can obtain favorable signal quality. In particular, to provide an optical recording medium that prevents deformation of the optically transparent substrate or the channels cut into the substrate used to guide the laser as well as prevent degradations in the recording signals due to deformation of the protective film on the reflecting film by making the diffusion of the thermal energy by the laser irradiation sufficient and/or by controlling the thermal decomposition characteristics of the dye material. In addition, an object of the present invention is to provide an optical recording medium that can favorably perform multi-level recording control.
The inventor has diligently conducted research on an optical recording medium and discovered a recording method that performs multilevel recording on this optical recording medium, and also verified that this recording method can perform high-density multilevel recording in five levels or more on this optical recording medium. Furthermore, the inventor has performed various types of experiments and found that stipulating the material properties of the optically transparent substrate and the coefficient of thermal conductivity and film thickness of the reflecting film was important for thermal diffusion. In addition, the inventor has found that if a dye whose thermal decomposition exceeds a range extending over 100xc2x0 C. or more is used, favorable recording becomes possible extending over the time when the laser irradiation time goes from short to long and/or extending over the time when the laser irradiation power goes from low to high. Even further, the inventor has found that if a dye whose thermal decomposition starts at 300xc2x0 C. or less is used, high sensitive recording becomes possible through which the inventor has completed the present invention.
In other words, the above-mentioned objectives are achievable by means of the present invention as follows.
(1) An optical recording medium, having a recording layer, whose principal component is a dye, provided on an optically transparent substrate, and a reflecting film formed on the recording layer, the medium being able to record information by irradiating a laser beam to form recording marks on said recording layer and read the recorded information by irradiating a reading laser beam onto the recording mark; wherein said recording layer has a virtual recording cell specified in an arbitrary unit length in a direction of relative movement between the laser beam and the recording layer and in a unit width in a direction that intersects said direction of movement at a right angle and continuously set in said direction of movement; said recording layer at said virtual recording cell can form recording marks with different sizes in correspondence to either a modulation of an irradiation time of the laser beam or an irradiation power in five levels or more; this allows multilevel recording of five levels or more of information by means of modulating the reflectance based on the area ratio of said recording marks to said virtual recording cells and transmittance, at least the area ratio; said optically transparent substrate is made of a thermoplastic resin with a glass transition point (Tg) of between 80xc2x0 C. and 160xc2x0 C.; said reflecting film is a metal with a coefficient of thermal conductivity of 300 k/Wxc2x7mxe2x88x921xc2x7Kxe2x88x921 or more and a film thickness at the recording mark of 50 nm or more.
(2) The optical recording medium according to (1), wherein said reflecting film material is characterized by the product of the coefficient of thermal conductivity and the film thickness being 2xc3x9710xe2x88x925 k/Wxc2x7Kxe2x88x921 or more.
(3) An optical recording medium, having a recording layer, whose principal component is a dye, provided on an optically transparent substrate, the medium being able to record information by irradiating a laser beam to form recording marks on said recording layer and read the recorded information by irradiating a reading laser beam onto the recording mark; wherein said recording layer has a virtual recording cell specified in an arbitrary unit length in a direction of relative movement between the laser beam and the recording layer and in a unit width in a direction that intersects said direction of movement at aright angle and continuously set in said direction of movement; aid recording layer at said virtual recording cell can form recording marks with different sizes in correspondence to either a modulation of an irradiation time of the laser beam or an irradiation power in five levels or more; this allows multilevel recording of five levels or more of information by means of modulating the reflectance based on the area ratio of said recording marks to said virtual recording cells and transmittance, at least the area ratio; the dye used in said recording layer is made in such a manner that differences in a weight reduction start temperature due to thermal decomposition (TG) and a temperature when a weight thereof becomes 20% of an original weight thereof extends over a range of 100xc2x0 C. or more.
Using a material with a coefficient of thermal conductivity of 300 k/Wxc2x7mxe2x88x921xc2x7Kxe2x88x921 as the reflecting film material in the present invention or setting the thickness of the reflecting film to 50 nm or less results in excessive heat causing deformation of the substrate or the channels cut into the substrate used to guide the laser as well as deformation of the protective film on the reflecting film leading to degradations in the recording signals.
This effect is dependent on the glass transition temperature of the substrate material on which laser guide grooves are provided. When a material with a high glass transition temperature, such as glass, is used as the material, deformation due to heat is not apparent. If a material with a glass transition temperature between 80xc2x0 C. and 160xc2x0 C. is used, this effect was found to be quite evident.
This thermal diffusion effect was also verified to be especially large during multilevel recording with adjacent recording marks in the recording direction.
These effects were present due to heat that occurred while recording in optical recording mediums used up to the present. However, it is assumed that even greater effects would more easily occur due to recording marks being placed adjacent in a linear direction in order to improve the recording density in the multilevel recording method.
Further, the size of the recording marks stated here refers to the size of changes in the quantity when the material constituting the recording layer decomposes and deteriorates due to irradiation of the laser beam to change the refractive index thereof or when the transmittance is changed due to the size in the direction of thickness when the refractive index changes.
In the present invention the reflectance could be controlled in many levels by means of specifying a dye constituting the recording layer.
Further, the optical recording medium may be constructed as follows.
(4) The optical recording medium according to (3), wherein said dye is characterized by weight reduction due to said thermal decomposition starting at 300xc2x0 C. or less and continuing until 350xc2x0 C. or more.
(5) the optical recording medium according to (3), wherein a reflecting film is provided on said recording layer on a side opposite to a side on which the laser beam is incident, said optically transparent substrate is made of a thermoplastic resin with a glass transition point (Tg) of between 80xc2x0 C. and 160xc2x0 C., and said reflecting film is a metal with a coefficient of thermal conductivity of 300 k/Wxc2x7mxe2x88x921xc2x7Kxe2x88x921 or more and a film thickness on a recording mark of 50 nm or more.
(6) The optical recording medium according to (5), wherein said reflecting film material is characterized by the product of the coefficient of thermal conductivity and the film thickness being xc3x9710xe2x88x925 k/Wxc2x7Kxe2x88x921 or more.
(7) The optical recording medium according to (4), wherein a reflecting film is provided on said recording layer on a side opposite to a side on which the laser beam is incident, said optically transparent substrate is made of a thermoplastic resin with a glass transition point (Tg) of between 80xc2x0 C. and 160xc2x0 C., and said reflecting film is a metal with a coefficient of thermal conductivity of 300 k/Wxc2x7mxe2x88x921xc2x7Kxe2x88x921 or more and a film thickness on a recording mark of 50 nm or more.
(8) The optical recording medium according to (7), wherein said reflecting film material is characterized by the product of the coefficient of thermal conductivity and the film thickness being 2xc3x9710xe2x88x925 k/Wxc2x7Kxe2x88x921 or more.
(9) The optical recording medium according to any one of (1)-(8), wherein the unit length of said virtual recording cell is set almost equal to the length of the recording mark formed by laser beam irradiation for the maximum amount of time.
(10) The optical recording medium according to any one of (1)-(9), wherein: grooves for guising laser beam are provided along said recording layer, said virtual recording cells are set inside said grooves and said unit width matches a width of said groove.
(11) The optical recording medium according to any one of (1)-(10), wherein said unit length in said virtual recording cells are equal to or less than the diameter of beam waist of said reading laser beam.
(12) The optical recording medium according to any one of (1)-(11), wherein information is recorded in multiple levels in advance on one part of said recording layer.
(13) The optical recording medium according to any one of (1)-(12), wherein specific information which represents a multilevel recording medium is recorded on at least one of said virtual recording cells and a multilevel recorded part.
(14) The optical recording medium according to any one of (1)-(13), wherein grooves for guiding laser beam are provided along said recording layer and are cut in the middle.